FIG. 22 illustrates a radiation image captured by a radiation image capturing apparatus. In the case of observing a lung field of a subject reflected in such a radiation image, it is necessary to perform image processing to improve the visibility of the lung field by adjusting the contrast of the lung field.
In a radiation image, various portions of a subject such as a bony part other than a lung field are reflected. A bony part of a subject is darkly reflected in a radiation image because the bony part hardly transmits radiation. Furthermore, the portion outside the contour of the subject reflected in the radiation image is a portion not reflecting the subject but a portion reflecting the air. The portion not reflecting the subject is brightly reflected in the radiation image because there is nothing to transmit radiation. In the radiation image, the lung field is brighter than the bony part of the object but darker than the portion outside the contour of the subject which is a portion reflecting the air.
The lung field of the radiation image totally looks to be filled with a gray color with poor contrasting density. This is because the pixels located in the lung field reflected in the radiation image have similar pixel values.
When a contrast adjustment is executed for the entire radiation image to increase the visibility of the lung field, the contrast adjustment is also executed for the portions of the radiation image in which the bony part and the air are reflected. Although such a contrast adjustment may increase the visibility of the radiation image as a whole, as far as the lung field is concerned, there is not so much improvement in visibility. The lung field after the contrast adjustment is still totally poor in contrasting density because of the following reasons. That is, in the case of expressing the contrast of the lung field, low pixel values are used to express bony parts of a subject, and high pixel values are used to express the portion where the air is reflected. Therefore, the lung field should be expressed with the remaining moderate pixel values.
Under the circumstances, a method of making a contrast adjustment only on a lung field in a radiation image has been conventionally conceived. According to this method, since the lung field can be expressed with more various color tones, the visibility of the lung field can be assuredly increased. The method is intended to execute trimming for extracting a lung field in a radiation image and make a contrast adjustment on the trimmed image in which the lung field is enlarged. In the trimmed image, dark portions reflecting bony parts of a subject and bright portions reflecting the air are excluded, so the trimmed image is not affected by these portions.
A conventional lung field trimming method will be described. In a conventional method, first, the edge enhancement processing is performed on a radiation image as shown in FIG. 23. The edge enhancement processing is image processing that can be realized by spatial processing such as differentiation processing and enhances the darkness of the portions where the pixel value in the image changes extremely. With this edge enhancement processing, the contour of the subject can be grasped. The edge enhancement is detailed in Patent Document 1.
According to the conventional method, the trimming processing of cutting out the region including a lung field as shown in FIG. 24 is performed based on the contour of the subject. In the trimming processing at this time, the lung field region is recognized based on the edge enhanced image, and the lung field including its periphery is roughly cut out from the radiation image.
Subsequently, the contour of the lung field is searched from the trimmed image as shown in FIG. 25. Searching the contour of the lung field is relatively easy since the trimmed image reflects almost only the lung field. In this way, the visibility of the lung field is improved. As described above, the recognition of the contour of the lung field is executed in two stages.
By performing the contrast adjustment only on the lung field surrounded by the searched lung field contour, the accuracy of the lung field can be assuredly improved.